Retailers often sell "AT&T Prepaid SIM Kits" in the electronics or prepaid phone section. Get a Replacement SIM - AT&T Wireless Customer Support
You can buy AT&T Prepaid (formerly GoPhone) SIM cards through official AT&T channels or at major third-party retailers. Most SIM kits are "3-in-1" (Standard, Micro, and Nano sizes) to fit any compatible device.